This invention relates to the production of alumina monohydrate. More particularly, this invention relates to the production of a pigment grade alumina monohydrate characterized by a uniform particle size.
It is known to produce alumina monohydrate by digestion in water or steam. It is also known that the presence of acid in this digestion is beneficial. For example, Tosterud U.S. Pat. No. 1,953,201, issued Apr. 3, 1934 and assigned to the assignee of this invention, produced a very finely divided alumina hydrate by subjecting alumina trihydrate to a digestion which usually comprises two steps involving a first digest in water, an alkaline solution, or simply heating the trihydrate followed by a subsequent digest in either an acidic or alkaline media. In one instance, the fine hydrate was produced from Bayer scale using a single treatment. Hughes U.S. Pat. No. 3,385,663 also produces hydrate by subjecting alumina trihydrate to an acid digest which results in the formation of a monohydrate so finely divided so as to form a colloidal dispersion in water.
Particles produced by these processes, however while undoubtedly useful in certain applications, are too small to have good optical properties in paper, paint, or ink.
Thibon et al U.S. Pat. No. 2,656,250 and French Pat. No. 1,008,828 describe a process for the production of fine particle size monohydrates using trihydrate seed formed by a precipitation process. Such preliminary processing of the trihydrate to obtain the finely divided seed particles adds additional cost to the process.